1. Field of the Invention
The present invention relates to an extreme ultraviolet (EUV) light source apparatus for generating extreme ultraviolet light by irradiating a target material with a laser beam.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress toward finer fabrication. In the next generation, microfabrication at 60 nm to 40 nm, further, microfabrication at 30 nm and beyond will be required. Accordingly, in order to fulfill the requirement for microfabrication at 30 nm and beyond, for example, exposure equipment is expected to be developed by combining an EUV light source for generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by irradiating a target with a laser beam, a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that light of only the necessary waveband can be radiated by selecting the target material, and that an extremely large collection solid angle of 2π to 4π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure such as electrodes surrounding the light source. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography, which requires power of more than 100 watts.
In the LPP light source, in the case where a solid material is used as a target to be irradiated with a laser beam in order to generate plasma, when a laser beam irradiation region is turned into plasma, heat generated by the irradiation with the laser beam is conducted around the laser beam irradiation region, and the solid material is melted around the laser beam irradiation region. The melted solid material becomes debris having diameters of several micrometers or more and is emitted in a large amount, and damages optical elements within a chamber such as an EUV collector mirror coated with a mirror coating and reduces its reflectance. Further, in the case where a liquid material is used as the target, scattered debris also damages the optical elements within the chamber. On the other hand, in the case where a gas is used as the target, the amount of debris becomes smaller, but the conversion efficiency from the power supplied to a drive laser to the power of EUV light becomes lower.
That is, in the case where a metal such as tin (Sn) is used as the target, the luminous efficiency of the EUV light having a wavelength of 13.5 nm required by exposure equipment is equal to or more than twice the luminous efficiency in the case where xenon (Xe) is used as the target. However, when tin is used as the target, the debris of tin scattered by the laser irradiation adheres to the EUV collector mirror, and the reflectance of the EUV light is reduced and the lifetime of the EUV collector mirror becomes shorter.
As a related technology, Japanese Patent JP-P3433151B discloses a laser plasma X-ray source that prevents damage on an optical mirror due to generated debris and has an improved collection efficiency of X-rays. The laser plasma X-ray source disclosed in JP-P3433151B includes a magnetic field applying unit for applying a magnetic field in a direction orthogonal to the injection direction of a target. According to JP-P3433151B, when it is assumed that the traveling direction of the debris before deflected by the magnetic field is the injection direction of the target, the optical mirror is provided in a direction in which the ion-state debris deflected by the magnetic field do not fly, and thereby, damage on the optical mirror can be prevented.
Further, Japanese Patent JP-P2552433B discloses a removing method and apparatus that can fundamentally remove debris produced from a solid target of a laser plasma X-ray source by using a relatively simple method. In the laser plasma X-ray source disclosed in JP-P2552433B, charge is provided to neutral particles produced together with X-rays from plasma on the surface of the target material by using ultraviolet light, and an electromagnetic field, in which an electric field and a magnetic field are orthogonal, is generated by using a pair of electrodes in mesh forms provided along a path of the X-rays and an electromagnet provided between the pair of electrodes. According to JP-P2552433B, charged fine particles are caused to pass through the electromagnetic field, and thereby, the tracks of the charged fine particles are curved and eliminated to the outside of the path of the X-rays. Thereby, X-ray optical elements provided in the path of the X-rays can be protected.
Furthermore, Japanese Patent Application Publication JP-P2006-80255A discloses means for ionizing a neutral particle target by irradiation with an X-ray, irradiation with light emitted from plasma, or irradiation with microwave in an extreme ultraviolet light source apparatus using a magnetic field.
In the following description of the present application, particles with no charge are defined as neutral particles. The neutral particles in the case of using a tin target include a vapor of metal tin, clustered metal tin with no charge, fine particles of metal tin with no charge, and so on.
If the flying tracks of the debris are changed by using the magnetic field as disclosed in JP-P3433151B, the neutral particles included in the debris have no charge and it is impossible to change the tracks of the neutral particles. If the technology is applied to an EUV light source apparatus, it is impossible to prevent the mirror damage (damage due to sputtering or adherence of debris).
Further, in JP-P2552433B, charge is provided to the neutral fine particles by providing an ultraviolet lamp for radiating ultraviolet light in the path from the plasma to an X-ray output window, but the path is limited to the substantially linear form, and the available solid angle of the X-rays are very limited. If the technology is applied to an EUV light source apparatus, there is a problem that the collecting efficiency of the EUV light becomes extremely lower.
Furthermore, according to the ionization mechanism disclosed in JP-P2006-80255A, the neutral particles within the chamber can be ionized and eliminated by the action of the magnetic field, but the X-rays and microwave have weak directivity and the efficiency of the reflection and collection using optical elements is not sufficient. Therefore, a method of performing ionization with higher efficiency is required.